1. Field of the Invention
The present invention relates to a decay heat removal system of a liquid metal reactor, and introduces a new heat exchange system which integrates a decay heat removal heat exchanger or decay heat exchanger (DHX) and an intermediate heat exchanger (IHX). The new heat exchange system makes it possible for effective decay heat removal to start immediately after an occurrence of an accident while maintaining the complete passivity of the decay heat removal operation. By this invention, passive, proper and stable cooling of the nuclear core can be achieved from the initial stage of an accident.
2. Description of Related Art
Liquid Metal Reactor
A liquid metal reactor (LMR) generates heat using fast neutrons from nuclear fission, and simultaneously converts a non-fissile material U238 into a fissile material Pu239, thereby serving as a breeding reactor by producing more fissile material than the fuel it consumes. Further, the liquid metal reactor is a reactor which can burn radioactive nuclides produced from other type reactors such as water-cooled reactors, and thus can reduce substantially the storage load of high level radioactive wastes generated from other type reactors.
The above liquid metal reactors are divided into loop type reactors and pool type reactors. The loop type reactor has a structure such that heat transfer devices of its primary heat transport system are installed outside a reactor vessel, and is advantageous in that the heat transfer devices are easily maintained and repaired and the reactor vessel has a simple structure. On the other hand, the pool type reactor has a structure such that its primary heat transport system including the equipment such as intermediate heat exchangers (IHXs) and pumps are installed in a reactor vessel, and is advantageous in that the leakage of the coolant due to the breakage of a pipeline of the primary system is prevented and a large amount of the coolant is contained in the primary system, thus having a high thermal inertia that makes the system transient speed slow and provides a long grace time in an accident.
The liquid metal reactor uses liquid metal as coolant, and preferably uses sodium (Na) having an excellent heat removal capacity as coolant.
Decay Heat Removal Type
Conventional liquid metal reactors use various types of decay heat removal systems for removing decay heat from the nuclear core in an accident. Hereinafter, a pool type reactor will be exemplarily described.
FIG. 1 is a longitudinal-sectional view of a conventional active decay heat removal system.
In FIG. 1, a nuclear core 11 installed in a reactor 10 heats sodium (Na) 17 and feeds the heated sodium 17 into a hot pool 18, which is positioned in the upper part of the reactor 10. The reactor includes conventional pumps 12 for circulating the liquid sodium. The sodium 17 in the hot pool 18 transfers its heat to intermediate heat exchangers (IHXs) 13, thus being cooled. The cooled sodium 17 is fed into the cold pool 19, which is positioned in the lower part of the reactor 10, and again enters the core 11. The IHXs 13 transfer heat thereof to a steam generation system (not shown), and the steam generation system generates steam, and then generates electricity.
A decay heat exchanger 14 is installed separately from the IHXs 13 in the hot pool 18 of the reactor 10, and a valve 15 is installed in the pipeline connected to the decay heat exchanger 14. The valve 15 serves to prevent heat loss to the outside through the decay heat exchanger 14 when the reactor 10 operates normally. That is, the valve 15 is closed when the reactor 10 operates normally, and is opened in an accident.
In the active decay heat removal system shown in FIG. 1, the switch valve 15 installed in the pipeline connected to the decay heat exchanger 14 needs to be opened in an accident in order to activate heat exchange with the external atmosphere. It means that an active decay heat removal system has weak safety features of requiring the operation of active devices such as a motor and valve 15 and also the supply of electric power from the outside for the operation of the valve 15.
Accordingly, instead of the above active decay heat removal system, there is required a passive decay heat removal system, in which removal of decay heat is automatically activated without relying on active devices.
FIG. 2 illustrates a conventional passive decay heat removal system. The structure of the passive decay heat removal system of FIG. 2 is the same as that of the active decay heat removal system of FIG. 1 in that a nuclear core 21, which is installed in a reactor 20, heats sodium (Na) 27 and feeds via pumps 22 the heated sodium 27 into a hot pool 28, which is positioned in the upper part of the reactor 20, and the sodium (Na) is cooled by exchanging heat in IHXs 23.
In an accident, the normal heat transfer path of the core-IHX-steam generation system is not credited and the sodium in the reactor is heated since the normal heat transfer path is no longer available, and the sodium expands. Consequently, the sodium level X1 in the hot pool 28 rises, and the sodium in the reactor 20 flows over the overflow slot 30. The overflowed sodium 27 directly contacts the wall 31 of a reactor vessel 30, thus transferring its heat to the wall 31 of the reactor vessel 30. The heat transferred to the wall 31 of the reactor vessel 30 is transferred to the air route 26 outside the reactor vessel 30 by radiation and convection heat transfer, and is then transferred to the air flowing in the air route 26 divided by an air separator 24. The air, to which the heat is transferred, continuously flows out to the atmosphere by virtue of the difference in its density along its path, that is, by natural convection. Cold external air is introduced into the reactor vessel 30 through the air path 26. The arrow 25 in the air path 26 represents the flow path of the air.
The above-described passive decay heat removal system is operated completely by the natural phenomena without relying on any operator action or any active device operation at an accident, thus being advantageous in that the reliability of the system operation is very high. However, it takes several hours for the sodium to overflow, that is, it takes several hours for the decay heat removal system to become fully functional and be able to remove the decay heat properly. During this period of time before the system becomes functional, proper heat removal from the reactor pool is not made and it is difficult for the natural circulation flow head to be built up. The flow head is the driving force of the natural circulation in the pool which cools the core. Consequently, the core cooling capability becomes low and the temperature of the nuclear fuel in the core can rise excessively high.
Summarizing the description, in a conventional passive decay heat removal system, the volume of the fluid in the reactor needs to be expanded substantially for the system to be able to remove decay heat properly, and the expansion of the fluid volume requires time and a rise of the pool temperature, and this feature results in weak safety features that the core cooling capability is not certain during the time period of the volume expansion and the temperature in the reactor may become unnecessarily high.